1. Field of the Invention
This invention relates to an induction motor control system and, more particularly, to an induction motor control system which enables an induction motor to continue running even when there is a large increase in regenerative energy, and which performs regenerative braking with excellent efficiency.
2. Description of the Prior Art
Induction motors are employed in a variety of industrial fields and can be applied to a variety of loads. Some induction motors undergo rapid and frequent increases and decreases in speed, while in others there are positive and negative changes in load torque, such as when used in raising and lowering an object via a hoist. Thus there is a requirement that an induction motor functioning as a driving source be operated so as to generate a positive torque as well as a braking torque.
A method of controlling the operation of induction motors which has recently come into fairly widespread use employs a variable voltage-variable frequency inverter. While no major problems are encountered in this method when operating the motor in the driving mode, one difficulty which does arise is the manner of dealing with the rotational energy possessed by the rotor of the motor when operating the motor in the braking mode at the time of a reduction in speed. Two exemplary methods of dealing with this rotational energy have been adopted in the prior art. In one method, the flow of current to the induction motor is interrupted for braking to permit the motor to slow down naturally owing to mechanical loss attributed to the load. In the other method, the slip which occurs at deceleration is suitably controlled and is allowed to dissipate within the motor. The first method, however, requires too much time to achieve the speed reduction and has a very poor control response, while the second method causes the motor to overheat to such an extent that it cannot endure frequent increases and decreases in speed.
In another method, the rotational energy of the rotor is dissipated by allowing a smoothing capacitor, inserted in the inverter circuitry mentioned above, to charge until the charged voltage exceeds a specified value, whereupon the capacitor is discharged through a braking resistor connected in parallel with the inverter circuit, thereby to dissipate the energy. This method is disadvantageous because it may lead to destruction of the apparatus if the smoothing capacitor is charged to an excessively large voltage, and because costs are raised since the braking resistor increases in size and expense in accordance with the size of the machine to be driven by the motor. Moreover, the method is undesirable in terms of enhancing efficiency because of the fact that the braking energy is wasted in the form of thermal loss.
Regenerative braking systems, as shown in FIGS. 1(A) and 1(B), have been proposed in an effort to improve upon the foregoing arrangements.
FIG. 1(A) is a circuit diagram showing an induction motor operation control apparatus of the regenerative braking type according to the prior art. The apparatus includes a three-phase induction motor 1, a full-wave rectifier 2 constructed of diodes D.sub.1 through D.sub.6 for rectifying the U, V and W phases of the AC input power, a regenerative circuit 3 having a thyristor bridge comprising thyristors S.sub.1 through S.sub.6, a smoothing circuit 4 having a smoothing capacitor C.sub.1, a variable voltage-variable frequency inverter 5 composed of transistors TA.sub.1 through TA.sub.6, a rectifier 6 comprising diodes D.sub.1 ' through D.sub.6 ', and a step-up transformer for boosting the power source voltage.
To control the induction motor 1 with this conventional arrangement, for example, to decelerate the motor, the commanded speed is reduced in magnitude so that the synchronous speed becomes smaller than the motor speed, giving rise to a negative slip condition. Accordingly, the motor operates in the regenerative braking region, with the result that the voltage induced in the motor is rectified by the rectifier 6, thereby raising the voltage on the DC line side. The smoothing capacitor C.sub.1, in order for it to exhibit the smoothing function, is charged to a voltage that is 1.3 to 1.4 times the AC power supply voltage even when the motor is operating in the normal driving mode. Nevertheless, when the induction motor is operated in the regenerative region, the smoothing capacitor C.sub.1 is charged to, and held at, an even higher voltage. For example, if the AC power supply voltage is 200 volts, the voltage to which the capacitor C.sub.1 is charged is raised to approximately 290 volts. When the firing of the regenerative thyristor bridge 3 is controlled under such conditions, commutation cannot take place and regenerative operation becomes impossible even though it may be possible to fire the thyristors because the AC power supply voltage is lower than the voltage on the side of the DC line. In other words, since the thyristors S.sub.1 through S.sub.6 are forward biased, a thyristor which has already fired cannot be turned off, making regeneration impossible. To avoid this problem, the step-up transformer 7 is inserted between the AC power supply and the thyristor bridge 3, and the circuitry is arranged in such a manner that there will always be intervals in which the AC power supply voltage is higher than the voltage on the DC line side, thereby assuring commutation of the thyristors S.sub.1 through S.sub.6 and enabling operation in the regenerative braking region. However, the apparatus that employs this system is large in size and high in price owing to the need for the step-up transformer 7 of a large capacity.
In view of the foregoing drawback, the Inventors have previously proposed a system, illustrated in FIG. 1(B), which dispenses with the step-up transformer. In the proposed system, two switching transistors TR.sub.1, TR.sub.2 are connected in series with a thyristor bridge 31 and are turned off when any of the thyristors S.sub.1 through S.sub.6 commutes. This switching action severs the thyristors S.sub.1 through S.sub.6 from the lines A, B. Thus, overcoming the forward biased state of the thyristors so that commutation is assured.
While the previously proposed system is extremely effective, it does not take into account a situation where the voltage of the smoothing capacitor C.sub.1 (FIG. 1(A)) may rise owing to a substantial increase in regenerative energy. That is, when the regenerative energy becomes large in magnitude, the voltage developed by the smoothing capacitor C.sub.1 rises and there is a gradual increase in the regenerative current I.sub.R. This can damage the switching transistors TR.sub.1 TR.sub.2 or the thyristors S.sub.1 through S.sub.6 if the regenerative current I.sub.R exceeds an allowable limit. When such an arrangement is applied to the system of FIG. 1(A), the voltage of the smoothing capacitor C.sub.1 is monitored and an alarm signal is issued with said voltage reaches a dangerous level, which is preset. The alarm signal completely halts the operation of the regenerative circuit 3 and transistor inverter 6, with the result that the motor can no longer run from that point onward.